Diaphragm pump with a functionally safe diaphragm position control

ABSTRACT

The present invention relates to diaphragm pump ( 1 ) having a delivery chamber ( 3 ) and a working chamber ( 5 ), wherein the working chamber can be or is filled with a hydraulic fluid and is operatively connected to a pressure generating device in order to apply an oscillating pressure to the hydraulic fluid, further comprising a diaphragm ( 11 ) having at least one diaphragm layer ( 13 ) and a diaphragm core ( 15 ), which separates the delivery chamber ( 3 ) and the working chamber ( 5 ) from each other, wherein the diaphragm ( 11 ) is or can be operatively connected to a diaphragm return device ( 17 ) comprising a pull rod ( 19 ), which applies or can apply a return force on the diaphragm ( 11 ) in the direction of the suction stroke position, and further comprising a storage chamber ( 21 ) for holding the hydraulic fluid, and wherein the working chamber ( 5 ) and the storage chamber ( 21 ) are connected to each other by means of a return flow channel ( 25 ) closed by means of a closure element ( 23, 23 ′), and wherein the closure element ( 23, 23 ′) is operatively connected to the diaphragm core ( 15 ) and the diaphragm return device ( 17 ), so that the return force and a pressure force counteracting the return force as a result of the fluid pressure in the working chamber ( 5 ) act on the closure element ( 23, 23 ′), and wherein, when a predetermined triggering force is exceeded as a sum of the return force and the pressure force on the closure element ( 23, 23 ′), the return flow channel ( 25 ) is opened.

The present invention relates to a diaphragm pump with a functionallysafe diaphragm position control.

Diaphragm pumps have a delivery chamber with a suction connection and apressure connection and a working chamber which is separated from thedelivery chamber by a diaphragm. In order to deliver a medium, thediaphragm is moved to and for in an oscillatory manner between a firstand a second position, in which the working chamber is filled with ahydraulic fluid to which an oscillating pressure is applied. The twopositions of the diaphragm are normally designated as the pressurestroke position and as the suction stroke position.

Usually, the pressure connection is connected to the delivery chambervia a pressure valve formed as a check valve, and the suction connectionis connected to the delivery chamber via a suction valve, likewiseformed as a check valve. During the movement of the diaphragm from thefirst to the second position, the so-called suction stroke, the volumeof the delivery chamber is enlarged, which means that the pressure inthe delivery chamber falls. As soon as the pressure in the deliverychamber falls below the pressure in a suction line connected to thesuction connection, the suction valve opens and medium to be deliveredis sucked into the delivery chamber via the suction connection. As soonas the diaphragm moves from the second position in the direction of thefirst position again (this is the so-called pressure stroke), the volumein the delivery chamber decreases and the pressure in the deliverychamber rises. The suction valve is closed in order to prevent themedium to be delivered from flowing back into the suction line. As soonas the pressure in the delivery chamber exceeds the pressure in apressure line connected to the pressure connection, the pressure valveis opened, so that the delivery medium in the delivery chamber can beforced into the pressure line.

The diaphragm itself can be preloaded in a sprung manner in thedirection of the suction stroke position. The diaphragm will alwaysassume a position in which the forces acting on the diaphragm cancel oneanother out. The forces generated by the fluid pressure in the deliverychamber and those generated by the sprung preload in the direction ofthe suction stroke position counteract the forces generated by the fluidpressure in the working chamber.

The application of an oscillating pressure to the hydraulic fluid thusleads to an oscillating movement of the diaphragm and, associated withthis, to an oscillating pumping process of the delivery fluid from thesuction line into the pressure line.

Hydraulically operated diaphragm pumps are preferably used in thedelivery of delivery fluids under high pressures, since, as a result ofthe hydraulic fluid, uniform loading of the diaphragm is carried out andthe latter thus has a long service life.

Here, the application of the oscillating pressure to the hydraulic fluidis normally carried out by means of a moving piston. It is possiblethat, in the event of severe contamination of the suction valve or flowpast the piston, the quantity of fluid in the working chamber willdeviate from the desired quantity. In this case, either too muchhydraulic fluid can be collected in the working chamber, so that thediaphragm is deflected beyond its pressure stroke position, or there canbe too little hydraulic fluid in the working chamber, so that thediaphragm cannot reach the pressure stroke position. In the first casethere is the danger of excessive loading of the diaphragm, which reducesits service life and can lead to damage. In the second case, thedelivery volume per stroke is undesirably reduced.

Solutions for this problem are described in the prior art, for exampleDE 10 2013 105 072 A1 discloses a solution according to which a movementof the diaphragm beyond the pressure stroke position opens a passage toa storage chamber of the hydraulic fluid, so that the pressure in theworking chamber is automatically reduced.

Ideally, the supply of the diaphragm pump should be ensured under allload and operating conditions, in order to avoid damage to the diaphragmpumps and in particular to the diaphragm itself. The problem here is, inparticular, faults which, for example, can occur as a result of a closedor contaminated suction line. In this case, there is too little or nodelivery fluid in the delivery chamber. As described, the forcesgenerated by the fluid pressure in the delivery chamber and the forcesgenerated by the sprung preload in the direction of the suction strokeposition counteract the forces generated by the fluid pressure in theworking chamber. However, if there is no or too low a quantity ofdelivery fluid in the delivery chamber, the diaphragm can be deflectedbeyond the pressure stroke position, since the fluid pressure in thedelivery chamber is too low.

To solve this problem, it is known, for example in the prior art, tofall back on powerful return springs. However, these have thedisadvantages that severe cavitation arises in the delivery chamber,which is associated with foam formation and the development of noise,and the diaphragm is subjected to extreme loadings, above all at theclamping points.

The object of the present invention is therefore to overcome thedisadvantages of the prior art and in particular to supply a diaphragmpump which prevents damage to the diaphragm in the event of a fault,with the development of noise and foam formation preferably beingprevented.

This object is achieved by a diaphragm pump having a delivery chamberand a working chamber, wherein the delivery chamber comprises a pressureconnection and a suction connection, and wherein the working chamber canbe or is filled with a hydraulic fluid and is operatively connected to apressure generating device in order to apply an oscillating pressure tothe hydraulic fluid, further comprising a diaphragm having at least onediaphragm layer and a diaphragm core, which separates the deliverychamber and the working chamber from each other and which can betransferred from a pressure stroke position into a suction strokeposition and back again, wherein the volume of the delivery chamber inthe pressure stroke position of the diaphragm is smaller than in thesuction stroke position, and wherein the diaphragm is or can beoperatively connected to a diaphragm return device comprising a pullrod, which applies or can apply a return force on the diaphragm in thedirection of the suction stroke position, and further comprising astorage chamber for holding the hydraulic fluid, and wherein the workingchamber and the storage chamber are connected to each other by means ofa return flow channel closed by means of a closure element, and whereinthe closure element is operatively connected to the diaphragm core andthe diaphragm return device, so that the return force and a pressureforce counteracting the return force as a result of the fluid pressurein the working chamber act on the closure element, and wherein, when apredetermined triggering force is exceeded as a sum of the return forceand the pressure force on the closure element, the return flow channelis opened. The invention is based on the surprising finding that, as aresult of the inventive design of a closure element operativelyconnected to the diaphragm core and the diaphragm return device, theforces acting on the diaphragm can be limited effectively to apredetermined value in that, when a triggering force is exceeded, areturn flow channel is opened and the fluid pressure in the workingchamber is reduced, in that the fluid can flow out of the workingchamber into the storage chamber. If two forces act in oppositedirections, then the result is that the magnitude of the overall forceis reduced by the fact that the greater force magnitude is reduced bythe smaller. However, this assumes that the forces act at the same pointof action. In the diaphragm pump according to the invention, the returnforce acts on or in the region of the end of the pull rod of thediaphragm return device located opposite the diaphragm, and the pressureforce acts in the region of the diaphragm. The points of action ofreturn force and pressure force are thus located, according to theinvention, on opposite sides of the closure element, so that theaddition of return force and pressure force acts on the closure elementitself. If, according to the invention, this sum exceeds a predeterminedtriggering force, the closure element is opened and the pressure forceis reduced. Thus, the diaphragm is protected against excessively highloads. The delivery pressure in the delivery chamber, by contrast,counteracts the pressure force directly, since both forces act directlyon the diaphragm on one side of the closure element. The pressure forceis therefore reduced in its magnitude by the force of the deliverypressure in the delivery chamber acting against the former. If thedelivery pressure in the delivery chamber falls abruptly, the magnitudeof the pressure force conversely rises sharply, so that the resultantsum of pressure force and return force can lie above the triggeringforce, this initially being independent of the deflection of thediaphragm between suction stroke position and pressure stroke position.However, provision can in particular be made that, in the event of adeflection of the diaphragm away from the suction stroke position beyondthe pressure stroke position, the closure element and the return flowchannel are opened. In order to generate the return force, according tothe invention spring elements are preferably used. If the diaphragm ismoved away from the suction stroke position beyond the pressure strokeposition, the return force of the spring element acting in the directionof the suction stroke position increases. At the same time, the pressureforce counteracting the return force, generated by the fluid in theworking chamber and acting on the diaphragm in the opposite direction,rises. By means of a suitable choice of the triggering force, themaximum deflection of the diaphragm can thus be determined. According toan inventive embodiment of the present invention, this can also be doneirrespective of the delivery pressure in the delivery chamber. Accordingto an embodiment of the present invention, it has proven to beparticularly advantageous if the return flow channel extends partly inthe diaphragm return device. According to the invention, the closureelement is operatively connected to the diaphragm core and the diaphragmreturn device. It has transpired here that the return flow channelparticularly suitably extends partly in the diaphragm return device, sothat an integrated solution of diaphragm core, diaphragm return device,closure element and return flow channel can be provided. Thisconfiguration has proven to be particularly compact and reliable. Inaddition, it may be preferred for diaphragm core and pull rod to bedetachably connected to each other, wherein, when the diaphragm core andpull rod are connected, the return flow channel is closed, and when theyare not connected, the return flow channel is opened. Provision can bemade where the closure element is formed by the diaphragm core.

According to this preferred embodiment, diaphragm core and pull rodtogether form the closure element. The two are detachably connected toeach other, wherein the connection is released only when the triggeringforce is exceeded. Once the connection has been released, the workingfluid can flow into the storage chamber through the return flow channelpreferably partly integrated into the pull rod, so that the pressure inthe working chamber decreases. According to an embodiment of the presentinvention, it may be preferable that the closure element comprises amagnet and/or is operatively connected thereto, wherein direction andintensity of the magnetic force correspond to the predeterminedtriggering force and, when the triggering force is exceeded the returnflow channel is opened. According to the invention, provision can bemade for the diaphragm core to comprise a magnet, which is operativelyconnected to the pull rod of the diaphragm return device and is designedand configured to keep the diaphragm core connected to the pull roduntil the triggering force has been exceeded. If the triggering force isexceeded, the diaphragm core is detached from the pull rod and the twoelements are present separately. In this state, the return flow channelpreferably integrated in the pull rod is opened and the working fluidcan flow into the storage chamber, so that the pressure in the workingchamber, and thus the pressure force, is reduced. A magnetic connectionaccording to the invention of diaphragm core and pull rod has inparticular the advantage that a reversible solution is provided. As soonas the triggering force is undershot, diaphragm core and pull rod can beconnected again and the diaphragm pump according to the invention cancontinue its operation without the diaphragm having been damaged. Ofcourse, provision can also be made for the magnet to be comprised by thepull rod, by another element of the diaphragm return device and/or afurther element of the diaphragm pump. Alternatively, provision can alsobe made where the closure element comprises an intended breaking pointas overload protection, which resists the triggering force until thelatter is exceeded and, when the triggering force is exceeded, breaksand the return flow channel is opened. Such an intended breaking pointas a constituent part of the connection of diaphragm head and pull rodprevents the same re-coupling and thus leads to the situation where, inthe event of a protective event occurring for the diaphragm, thecorresponding component must be replaced before re-commissioning.Provision can also be made where the working chamber is arranged in ahousing, wherein the return flow channel extends partly through thehousing in the region of the pull rod. The working chamber of adiaphragm pump according to the invention is usually separated from thestorage chamber by means of a housing. To provide the return flowchannel which makes it possible for the working fluid to flow from theworking chamber into the storage chamber, said channel must necessarilylead through the aforesaid housing. According to the invention, it hasproven to be particularly positive if the return flow channel whichextends partly in the pull rod is continued in the region of the housingadjacent to the pull rod. As a result, it is possible to dispense withhoses, lines and the like, so that an efficient and reliable solution isprovided.

It may be particularly preferred that the diaphragm return device isguided partly through a region of the housing that is formed as a guidesection, wherein the return flow channel has on its side facing thediaphragm return device an elongated connecting section along thedeflection of the diaphragm return device, so that, irrespective of thedeflection position of the diaphragm return device, the section of thereturn flow channel that is located in the diaphragm return device andthe section of the return flow channel that is located in the housingare operatively connected.

Such a configuration of a return flow channel according to the inventionmakes it possible that no hoses, lines or the like are needed for aconnection of the section of the return flow channel that is arranged inthe pull rod and the section of the return flow channel that is locatedin the housing. Instead, irrespective of the deflection of the diaphragmreturn device, a secure connection of the section of the return flowchannel extending in the latter to the stationary section which extendsin the housing is made possible. Furthermore, provision can be made forthe pull rod to be operatively connected to a spring element, so thatthe diaphragm is sprung preloaded in the direction of the suctionstroke. It has proven to be advantageous, to provide the return force,to fall back on a spring element, which is connected to the end of thepull rod opposite the diaphragm. It may in particular be advantageousthat the deflection of the pull rod is limited by means of a fixed stop,so that the pull rod cannot be moved from the suction stroke positionbeyond the pressure stroke position or a predetermined position furtherremoved from the suction stroke position than the pressure strokeposition. This has the advantage in particular that, in the event of anoverfilling that occurs slowly, for example as a result of anincreasingly contaminated suction line, a maximum deflection of thediaphragm can be defined. If the diaphragm is deflected beyond thispoint, the sum of return force and pressure force rises significantlyand the return flow channel is opened. It has also proven to beadvantageous that the working chamber and the storage chamber areconnected to each other via a return flow channel closed by a furtherclosure element, wherein the further closure element is connected to thepull rod so as to be movable relative to the latter, so that the furtherclosure element can be transferred from a closed position into an openposition and back, and wherein the further closure element comprises aforce generating element and/or is operatively connected to the latter,which locks the further closure element in the closed position, andwherein the further closure element is transferred into the openposition and the further return flow channel is opened when it is trueof the pressure difference between the pressure in the storage chamberp₂ and the pressure in the working chamber p₁ that p₂−p₁>a, where a is apredetermined pressure.

Provision can be made where the further return flow channel extendspartly in the diaphragm return device, in particular in the pull rod,and wherein the further return flow channel is preferably connected tothe return flow channel.

It is thus possible to ensure that, in the event of a fluid loss in theworking chamber, fluid can be topped up from the storage chamber as soonas the pressure in the working chamber falls below a predeterminedvalue.

It is particularly advantageous that, according to the invention, theleakage compensation is integrated directly in the diaphragm returndevice and can be operatively connected to the same return flow channel,so that the number of apertures through the housing is minimized. Eachconnection extending through the housing is associated with the risk ofa lack of tightness, in particular when there are high pressures in theworking chamber, so that a housing that is closed as far as possible isfundamentally preferred.

Further features and advantages of the invention can be gathered fromthe following description, in which exemplary embodiments of theinvention will be explained by way of example by using schematicdrawings, without restricting the invention as a result.

Here:

FIG. 1: shows a lateral sectional view of an embodiment of a diaphragmpump according to the invention;

FIG. 2: shows a lateral sectional view of the embodiment of a diaphragmpump according to the invention according to FIG. 1;

FIG. 3: shows a further lateral sectional view of an embodiment of adiaphragm pump according to the invention according to FIGS. 1 and 2;

FIG. 4: shows a lateral sectional view of an alternative embodiment of adiaphragm according to the invention; and

FIG. 5: shows a lateral sectional view of a further embodiment of adiaphragm pump according to the invention.

In FIG. 1, by way of example, an embodiment of a diaphragm pump 1according to the invention, having a delivery chamber 3 and a workingchamber 5, is shown. The delivery chamber 3 has a pressure connection 7and a suction connection 9. The delivery chamber 3 is separated from theworking chamber 5 by means of a diaphragm 11.

The working chamber 5 is filled with a hydraulic fluid and isoperatively connected to a pressure generating device, not shown, inorder to apply an oscillating pressure to the hydraulic fluid.

The diaphragm 11 has at least one diaphragm layer 13 and a diaphragmcore 15, wherein the diaphragm 11 can be transferred from a pressurestroke position into a suction stroke position and back again.

As shown in FIG. 2, the volume of the delivery chamber 3 in the pressurestroke position of the diaphragm 11 is smaller than in the suctionstroke position. The usual deflection of the diaphragm is identified byD.

The diaphragm 11 is additionally connected to a diaphragm return device17, comprising a pull rod 19 which applies a return force on thediaphragm 11 in the direction of the suction stroke position.

Furthermore, a storage chamber 21 for holding the hydraulic fluid isshown, wherein the working chamber 5 and the storage chamber 21 areconnected to each other by means of a return flow channel 25 closed bymeans of a closure element 23.

As can be seen in FIG. 3, the diaphragm core 15 and the pull rod 19 aredetachably connected to each other, wherein, when the diaphragm core 15and pull rod 19 are connected, the return flow channel 25 is closed and,when they are not connected, the return flow channel 25 is opened, sothat in the embodiment of the present invention shown in FIGS. 1 to 3,the diaphragm core 15 together with the pull rod 19 forms the closureelement 23, wherein a secure connection is provided by means of a magnet27.

The magnet 27 is designed and configured to maintain the connectionbetween diaphragm core 15 and pull rod 19 until a triggering force isexceeded. The connection of diaphragm core 15 and pull rod 19, whichtogether form the closure element, is acted on firstly by the returnforce R, which acts through the spring element 39 on the end of the pullrod 19 opposite the diaphragm. The return force R is counteracted by themagnitude of the pressure force D and the delivery pressure F of thedelivery fluid in the delivery chamber 3. The pressure force D acts onthat end of the pull rod 19 which faces the diaphragm 11. Thus, the sumof pressure force D and return force R acts on the connection ofdiaphragm core 15 and pull rod 19.

Furthermore, it is shown in FIGS. 1 to 3 that the diaphragm returndevice 17 is guided partly through a region of the housing formed as aguide section 35. In this guide section 35, the return flow channel 25has on its side facing the diaphragm return device 17 an elongatedconnecting section 37 along the deflection of the diaphragm returndevice 17.

This elongated connecting section 37 is used for the purpose that,irrespective of the deflection position of the diaphragm return device17, the section of the return flow channel 25 that is located in thediaphragm return device 17 and the section of the return flow channel 25that is located in the housing are operatively connected, so that at anytime the connection between the working chamber 3 and the storagechamber 21 can be produced.

An alternative embodiment of the present invention is illustrated inFIG. 4, differing from the embodiment according to FIGS. 1 to 3 in thatto connect diaphragm core 15 and pull rod 19, recourse is made to anintended breaking point 33.

A further embodiment is shown in FIG. 5. In the embodiment shown in FIG.5, a leak compensating device is added. For this purpose, the workingchamber 5 and the storage chamber 21 are connected to each other via areturn flow channel 43 closed by a further closure element 41. Thefurther closure element 41 is connected to the pull rod 19 so as to bemovable relative thereto, so that the further closure element 41 can betransferred from a closed position into an open position and back.

The further closure element 41 is operatively connected to a forcegenerating element 45 which, in the embodiment according to FIG. 5, isin the form of a spring element. The further closure element 41 islocked in the closed position and is transferred into the open position,so that the further return flow channel 43 is opened when it is true ofthe pressure difference between the pressure in the storage chamber p₂and the pressure in the working chamber p₁ that p₂−p₁>a, where a is apredetermined pressure.

The features of the invention described in the preceding description,the claims and the drawings can be important, both individually and alsoin any desired combination, for the implementation of the invention inits various embodiments.

1. A diaphragm pump (1) having a delivery chamber (3) and a workingchamber (5), wherein the delivery chamber (3) comprises a pressureconnection (7) and a suction connection (9), and wherein the workingchamber can be or is filled with a hydraulic fluid and is operativelyconnected to a pressure generating device in order to apply anoscillating pressure to the hydraulic fluid, further comprising adiaphragm (11) having at least one diaphragm layer (13) and a diaphragmcore (15), which separates the delivery chamber (3) and the workingchamber (5) from each other and which can be transferred from a pressurestroke position into a suction stroke position and back again, whereinthe volume of the delivery chamber (3) in the pressure stroke positionof the diaphragm (11) is smaller than in the suction stroke position,and wherein the diaphragm (11) is or can be operatively connected to adiaphragm return device (17) comprising a pull rod (19), which appliesor can apply a return force on the diaphragm (11) in the direction ofthe suction stroke position, and further comprising a storage chamber(21) for holding the hydraulic fluid, and wherein the working chamber(5) and the storage chamber (21) are connected to each other by means ofa return flow channel (25) closed by means of a closure element (23,23′) [23′ is not yet found in the figures], wherein the closure element(23, 23′) is operatively connected to the diaphragm core (15) and thediaphragm return device (17), so that the return force and a pressureforce counteracting the return force as a result of the fluid pressurein the working chamber (5) act on the closure element (23, 23′), andwherein, when a predetermined triggering force is exceeded as a sum ofthe return force and the pressure force on the closure element (23,23′), the return flow channel (25) is opened, wherein diaphragm core(15) and pull rod (19) are detachably connected to each other, wherein,when the diaphragm core (15) and pull rod (19) are connected, the returnflow channel (25) is closed, and when they are not connected, the returnflow channel (25) is opened.
 2. The diaphragm pump as claimed in claim1, wherein, in the event of a deflection of the diaphragm (11) away fromthe suction stroke position beyond the pressure stroke position, theclosure element (23, 23′) and the return flow channel (25) are opened.3. The diaphragm pump as claimed in claim 1, wherein the return flowchannel (25) extends partly in the diaphragm return device (17). 4.(canceled)
 5. The diaphragm pump as claimed in claim 1, wherein theclosure element (23, 23′) is formed by the diaphragm core (15).
 6. Thediaphragm pump as claimed in claim 1, wherein the return flow channel(25) extends partly in the pull rod (19) of the diaphragm return device(17).
 7. The diaphragm pump as claimed in claim 1, wherein the closureelement (23, 23′) comprises a magnet (27) and/or is operativelyconnected thereto, wherein direction and intensity of the magnetic forcecorrespond to the predetermined triggering force and, when thetriggering force is exceeded, the return flow channel (25) is opened. 8.The diaphragm pump as claimed in claim 1, wherein the closure element(23, 23′) comprises an intended breaking point (33) as overloadprotection, which resists the triggering force and, when the triggeringforce is exceeded, breaks and the return flow channel (25) is opened. 9.The diaphragm pump as claimed in claim 1, wherein the working chamber isarranged in a housing, wherein the return flow channel (25) extendspartly through the housing in the region of the pull rod (19).
 10. Thediaphragm pump as claimed in claim 9, wherein the diaphragm returndevice (17) is guided partly through a region of the housing that isformed as a guide section (35), wherein the return flow channel (25) hason its side facing the diaphragm return device (17) an elongatedconnecting section (37) along the deflection of the diaphragm returndevice (17), so that, irrespective of the deflection position of thediaphragm return device (17), the section of the return flow channel(25) that is located in the diaphragm return device (17) and the sectionof the return flow channel (25) that is located in the housing areoperatively connected.
 11. The diaphragm pump as claimed in claim 1,wherein the pull rod (19) is operatively connected to a spring element(39), so that the diaphragm (11) is spring-loaded in the direction ofthe suction stroke.
 12. The diaphragm pump as claimed in claim 1,wherein the deflection of the pull rod (19) is limited by means of afixed stop, so that the pull rod (19) cannot be moved from the suctionstroke position beyond the pressure stroke position or a pre-determinedposition further removed from the suction stroke position than thepressure stroke position.
 13. The diaphragm pump as claimed in claim 1,wherein the working chamber (5) and the storage chamber (21) areconnected to each other via a return flow channel (43) closed by afurther closure element (41), wherein the further closure element (41)is connected to the pull rod (19) so as to be movable relative to thelatter, so that the further closure element (41) can be transferred froma closed position into an open position and back, and wherein thefurther closure element (41) comprises a force generating element (45)and/or is operatively connected to the latter, which locks the furtherclosure element (41) in the closed position, and wherein the furtherclosure element (21) is transferred into the open position and thefurther return flow channel (43) is opened when it is true of thepressure difference between the pressure in the storage chamber p₂ andthe pressure in the working chamber p₁ that p₂−p₁>a, where a is apredetermined pressure.
 14. The diaphragm pump as claimed in claim 12,wherein the further return flow channel (43) extends partly in thediaphragm return device (17), in particular in the pull rod (19), andwherein the further return flow channel (43) is preferably connected tothe return flow channel (25).